In the resistive SCFCL, the super conductor is directly connected in series with the line to be protected. To keep it superconducting, it is usually immersed in a coolant that is chilled by a refrigerator. Current leads are designed to transfer as little heat as possible from the outside to the coolant.
In normal operation, the current and its magnetic field can vary but temperature is held constant. The cross section of super conductor is such as to let it stay below critical current density, since its receptivity is zero in this regime; the impedance of the SCFCL is negligible and does not interfere with the network. All the same the superconductor’s impedance is truly zero only for dc currents. The more common as applications are affected by two factors. First, the finite length of the conductor produces a finite reactance which however can be kept low by special conductor architecture. Second a superconductor is not loss free in ac operation, the magnetic as field generated by the current produces so called ac losses basically, just eddy current losses. These are heavily influenced by the geometry of the conductor and can be reduced by decreasing the conductor dimension transverse to direction of local magnetic field. They barely contribute to total SCFCL impedance but dissipate energy in superconductor, thus raising cooling costs.
In case of a fault the inrush of current and magnetic field take the super conductor into the transition region, between zero resistance and normal receptivity. The fast rising resistance limits the fault current to a value some where between the nominal current and what ever fault current otherwise would ensure. After some time, perhaps a tenth of seconds, a breaker will interrupt the current.
The behavior of resistic fault current limiter is largely determined by the length of the superconductor and the type of material used for it.
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